The present invention relates to flex circuits for connecting magnetic heads to read and write circuits of a magnetic recording device. More particularly, the present invention relates to a flexible trace interconnect array for a multi-channel tape head which manifests reduced inter-channel cross talk as well as controlled electrical impedance characteristics.
Magnetic tape drives are typically employed to provide data backup and archival storage for user data records and programs. For digital data storage applications, tape drives typically employ either rotating heads, or non-rotating heads. One form of non-rotating head is the streaming tape drive. In a streaming tape drive multiple blocks of user data are typically written to tape in a single streaming operation, rather than in a series of start-stop operations of the tape transport. In the streaming tape drive, a magnetic tape head includes at least one read/write element. The head is typically positioned laterally relative to the tape path by a lead screw, which is controllably rotated by e.g. a stepper motor, or an equivalent arrangement. In this manner a single transducer element, or several spaced-apart elements, may write to, and read from, a multiplicity of linear tracks defined along the magnetic recording tape.
In order to permit the head to be moved laterally across the tape in order to confront the multiple parallel tape tracks, a flexible head interconnect arrangement is needed to connect the read/write elements of the head to electronic circuitry conventionally mounted on one or more printed circuit boards affixed to the tape drive base or housing. In the past, flexible wires, twisted together into pairs and gathered into a cable, have been employed as tape head interconnects.
Digital linear magnetic tape drives are an improved type of linear streaming magnetic tape drives. One well established digital linear magnetic tape drive is provided by Quantum Corporation as the DLT-7000 drive. This particular tape drive uses a single reel tape cartridge that supplies a stream of half-inch-wide tape via a leader and buckling mechanism. The Quantum DLT7000 tape drive has a four-channel head, with eight write elements and four read elements. A first set of four write elements are placed on one side of the four read elements, and a second set of four write elements are placed on an opposite side of the first set. This particular arrangement enables four data tracks to be written and then read-checked during a single forward tape streaming operation, and a second four data tracks to be written and read-checked during a single reverse tape streaming operation. Azimuth recording is employed to reduce cross talk between adjacent tape tracks. Therefore, the head is not only displaced laterally relative to the tape path, it is also rotated to a forward azimuth angle for forward direction, and rotated to a reverse azimuth angle for reverse direction data recording. Backward compatibility is achieved by orienting the head at a right angle to the tape path such that two non-azimuthal tracks may be simultaneously written and/or read during each tape streaming operation.
A flex circuit supporting the eight write elements and the four read elements of the Quantum DLT7000 tape drive product is described in commonly assigned U.S. Pat. No. 5,862,014 to Nute, entitled: xe2x80x9cMulti-Channel Magnetic Tape Head Module Including Flex Circuitxe2x80x9d, the disclosure thereof being incorporated herein by reference. The described flex circuit permitted the tape head freely to be laterally displaced and also to be rotated to the variously available azimuthal and linear tape confronting positions. In the arrangement described in the ""014 patent, approximately 128 linear data tracks were provided on the half-inch recording tape.
Data rates and track densities are increasing. One way to increase data rate of a magnetic recording system is to increase the write frequency. Another way to increase data rate is to increase the number of parallel write and read elements of the head and data channels of the tape drive so that more tracks are simultaneously written during each tape streaming operation. A third way to increase data rates is to employ partial response, maximum likelihood signaling techniques of the type known in magnetic disk drives.
One way to increase track density is to reduce linear track width and spacing by aligning the write elements/read elements closer together. By employing thin film inductive write elements and magneto-resistive read elements, it is practical to increase the number of data tracks of a one-half inch magnetic recording tape from e.g., 128 tracks to e.g., 1000 or more tracks. Since the head carrying the write and read elements must still be displaced laterally relative to the tape path, a flexible interconnect arrangement is needed in order to connect the write and read elements of the movable head to write and read electronics affixed to the printed circuit board of the drive electronics.
A flex trace interconnect array is preferred, because it affords the opportunity to control the electrical impedance characteristics of the traces, as taught for example by commonly assigned U.S. Pat. No. 5,737,152 to Balakrishnan, entitled: xe2x80x9cSuspension with Multi-Layered Integrated Conductor Trace Array for Optimized Electrical Parametersxe2x80x9d, the disclosure thereof being incorporated herein by reference. Commonly assigned U.S. Pat. No. 5,754,369 to Balakrishnan, entitled: xe2x80x9cHead Suspension with Self-Shielding Integrated Conductor Trace Arrayxe2x80x9d, shows an arrangement wherein a read conductor pair is interleaved between two conductors of a write conductor pair in a disk drive flexible trace interconnect. (In disk drive operations, simultaneous writing and reading operations are not usually present, and thus the write traces offer a measure of shielding to the read traces during disk drive data reading operations). The disclosure of the ""369 patent is also incorporated by reference herein.
Conventionally, the trace conductors connecting the preamplifiers to the read elements of the tape head are interleaved with the conductors connecting the write drivers to the write elements. Because of space restrictions, and the desire to reduce the interconnect mass, the trace conductors have to be placed close to each other. While it is desirable from an electrical viewpoint to space the conductors of any single channel as close to each other as possible, it is equally desirable to increase the spacing between adjacent conductors of separate channels.
FIG. 1 and FIG. 3A show a conventional layout of flex conductors on a flexible trace interconnect array 10 which connects two inductive write elements 12 and 14 and two magneto-resistive read elements 13 and 15 of a two-channel tape head 16 to a two-channel preamplifier/write driver circuit 18 of the tape drive. In this example one tape channel (track) is defined by write element 12 and read element 13, and another tape channel (track) is defined by write element 14 and read element 15. Further, in the FIG. 1 simplified example a conductor pair 20A-20B of flex interconnect 10 connecting read transducer 15 to its preamplifier in circuit 18 is interleaved between a conductor pair 22A-22B connecting write transducer 12 to its write driver in circuit 18 and a conductor pair 24A-24B connecting write transducer 14 to its write driver in circuit 18.
Flexible trace interconnects are presently available having trace widths as narrow as 75 xcexcm (approximately 3 mils). Thus, in the FIG. 1 multi-channel flex interconnect 10 a flex trace interconnect conductor geometry would have a cross-sectional layout of traces formed on a flexible substrate 11 as shown in FIG. 3A: - - - 75 xcexcm read trace 26A - - - 75 xcexcm inter-conductor space - - - 75 xcexcm read trace 26B - - - 75 xcexcm inter-pair space - - - 75 xcexcm write trace 22A - - - 75 xcexcm inter-conductor space - - - 75 xcexcm write trace 22B - - - 75 xcexcm inter-pair space - - - 75 xcexcm read trace 20A - - - 75 xcexcm inter-conductor space - - - 75 xcexcm read trace 20B - - - 75 xcexcm inter-pair space - - - 75 xcexcm write trace 24A - - - 75 xcexcm inter-conductor space - - - 75 xcexcm write trace 24B, etc. The conductors of trace pairs 20, 22, 24, and 26 could be widened or thickened, or both, if required for electrical reasons.
Each conductor in the FIG. 1 electrical schematic diagram has a resistance, an inductance and a capacitance associated with it. An equivalent circuit for three of the conductor pairs of FIG. 1 is shown in FIG. 2. The FIG. 2 equivalent circuit shows the pair of write-channel conductors 22A, 22B routed between the adjacent conductors 20B and 26A of the two read circuits. There are a number of factors that need to be considered. At higher write current frequencies, the resistance and inductance parameters are governed by skin-effect and proximity-effect phenomena. The resistance is affected by the current distribution in the trace conductors, which tends to move to the surface of each trace conductor at high frequencies. This movement of the current distribution effectively results in a smaller conductor cross section available for the current to flow through, and results in an increase in the high frequency electrical resistance of the trace conductor. The current-density vector and the magnetic flux follow the same movement toward the conductor surface. As the current moves towards the surface, so does the magnetic flux, which for the same current means lesser flux links the conductor. Thus, the electrical inductance of the trace conductor goes down with increasing frequency. The inter-trace capacitances are relatively frequency-independent and may be treated as constant values.
In addition to the parameters discussed above, the FIG. 2 equivalent circuit also includes elements that couple the trace conductors 22A and 22B of the write circuit with the conductors 26B and 20A of the two adjacent read circuits. This coupling is shown in FIG. 2 as coupling capacitors CCFEM.x. The mere presence of the adjacent conductors also affects the flux distribution of the current in the current-carrying conductors. Since this is a frequency-dependent phenomenon, different signals at different frequencies are affected to varying degree.
Another impact of the write conductors 22A and 22B on the read conductors 26B and 20A is due to the fact that the write current amplitude is approximately 40 to 60 milliamperes, whereas the read signals are on the order of one to a few millivolts. During simultaneous writing/read-checking operations of the tape drive, any signals induced by the write current on adjacent read conductors can couple into the read signal at the preamplifier.
The cross-coupling of write and read conductive traces and the disparity in write current levels to read signal levels suggests that the read and write signals need to be decoupled.
The present invention solves this problem within a flexible trace interconnect array for a multi-channel recording and playback head, such as a digital linear magnetic tape head.
One object of the present invention is to provide a multi-channel flexible trace interconnect array of trace pairs in which each trace pair is electrically decoupled from adjacently located trace pairs along the trace array.
Another object is to reduce cross talk and eddy current induction between pairs of conductive traces formed on a flexible circuit substrate used to interconnect transducer elements of a positionable head with write driver and read preamplifier circuitry non-moveably affixed to a base of a data storage device, such as a tape drive.
In accordance with one aspect of the principles of the present invention, a flexible multi-channel trace interconnect array is provided. The array has a head end for electrically connecting write and read trace pairs respectively to write and read elements of a data transducer head. The array includes a body formed of a flexible dielectric material which carries the write and read trace pairs and leads to a circuit connection end for connecting the write and read trace pairs respectively to write driver and read preamplifier circuits of a data storage device, such as a multi-channel tape drive. Each trace pair comprises two trace conductors, and each conductor has a defined trace width, such as 50 xcexcm. The conductors of each pair are separated along the body by an inter-conductor space having a defined width such as 50 xcexcm. Adjacent trace pairs are separated by an inter-pair space having a defined width which is greater than the defined trace width and greater than the defined inter-conductor space, such as 150 xcexcm-400 xcexcm, or more. Preferably, the inter-pair space has a defined width dimension lying in a range from approximately two to twenty times the inter-conductor defined width.
In one preferred arrangement multiple read trace pairs, such as at least twelve read trace pairs, are interleaved with multiple write trace pairs, such as at least twelve write trace pairs, along the trace array body.
In an alternative preferred arrangement, multiple write trace pairs are separated into a write pair group and multiple read trace pairs are separated into a read pair group. In this arrangement the write pair group is substantially spaced away from the read pair group along the array body. In a related preferred arrangement trace array body is divided into two elongated segments including a write group segment carrying the write group trace pairs, and a read group segment carrying the read group pairs. In a further related preferred arrangement the write group segment includes a portion leading to the write group circuit connection end which diverges away, preferably perpendicularly from the body and in an opposite direction of divergence away from the body of a portion of the read group segment leading to the read group circuit connection end. Preferably, although not necessarily, the write group segment portion has a length which is approximately equal to a length of the read group segment portion.
Furthermore, in this alternative preferred arrangement, each write element and a corresponding read element of the head are aligned to write and read a single storage track of a data storage medium such as magnetic tape. Accordingly, the head end of the array comprises a pattern of plate-through vias and bridging traces formed on an opposite side of the flex body so that a write pair connection location to a write element of the head is placed adjacent to a read pair connection location to the corresponding read element of the head.
These and other objects, advantages, aspects, and features of the present invention will be more fully appreciated and understood upon consideration of the following detailed description of preferred embodiments presented in conjunction with the accompanying drawings.